A solvent extraction process is one of the known processes for separating and recovering arsenic from an aqueous solution containing 5 valent arsenic (hereinafter, referred to as arsenic). Examples of the extracted include tributyl phosphate (TBP), trioctyl phosphate (TOP), dibutyl butyl phosphonate (DBBP), and di-2-ethylhexyl 2-ethylhexyl phosphonate.
The above described extractant is used alone or, if necessary, as a mixture thereof with a diluent such as mineral oil, etc., or with another extractant, as the so-called organic solvent.
Arsenic extracted with such organic solvents is generally subjected to stripping with acid, alkali or water, etc., and the arsenic in the solution after the stripping is recovered as arsenous acid by condensation or reduction.
One known process is a solvent extraction process for producing arsenic using TBP as an exrtractant for the arsenic containing sulfuric acid solution. This process comprises using a large amount of water for washing sulfuric acid in the arsenic containing solvent in a scrubbing step before stripping of arsenic from the arsenic containing solvent, thereafter carrying out multistage stripping of arsenic with water, heating the resulting solution after the stripping to evaporate water, by which arsenic is condensed, and blowing a sulfurous acid gas as the reducing agent through the condensed arsenic solution to crystallize arsenous acid. This process has many problems. For example, it has many complicated steps and it is very difficult to maintain the water balance due to the use of a large amount of water for washing or stripping. Furthermore, a great deal of energy is required for evaporating a large amount of water.
The present inventors have investigated possible solutions of the above described problems. As a result of these investigations, the inventors have found that arsenic acid is directly formed by bringing the arsenic containing organic solvent into contact with a reducing agent and water or an aqueous solution.